Ultrasonic sensors are used to make remote distance measurements. One particular use of ultrasonic sensors is within a vehicle occupant restraint system within a vehicle.
One particular type of restraint system includes an actuatable restraint device. Examples of actuatable devices include inflatable air bags and seat belt system pretensioners. The actuatable devices are actuated in response to one or more conditions for which it is predetermined that the occupant is to be restrained. For example, the restraint system includes a crash sensor that senses a vehicle collision. A controller of the restraint system causes actuation of the restraint device in response to a signal indicative of a vehicle collision from the crash sensor.
It is known in the art to adjust or tailor the actuation or deployment of a restraint device. For example, the art has recognized that it is not always desirable to inflate an air bag with 100 percent of the available gas provided from a source of inflation fluid. It is known to adjust or tailor the restraint deployment based upon one or more sensed occupant characteristics. An occupant restraint device that has an adjustable aspect that is adjusted in response to a determination based upon a sensed occupant characteristic is commonly referred to as a "smart restraint." It is known to use one or more ultrasonic sensors to sense one or more occupant characteristics (e.g., occupant position) for use in determining adjustment of a restraint device.
Ultrasonic sensors typically have a piezoelectric ceramic transducer that converts an excitation electrical signal into ultrasonic energy bursts. The energy bursts travel from the ultrasonic sensor, bounce off objects, and are returned toward the sensor as echoes. The transducer converts the echoes into electrical signals.
The efficiency of an ultrasonic transducer varies with the frequency at which the transducer is excited. It is known that ultrasonic sensors perform optimally at their natural resonant frequency. The natural resonant frequency is also referred to as the "ringdown" frequency, because once excitation of the transducer ceases, the transducer has a decaying ringdown that occurs at the natural resonant frequency of the transducer.
The natural resonant frequency (or natural frequency) of an ultrasonic sensor transducer changes or "drifts" in response to temperature change. In order to maintain an ultrasonic sensor operating at peak efficiency, it is known to adjust the operating or excitation frequency to track the changes in the natural frequency. One known method of tracking the "drifting" natural frequency includes using a temperature sensor co-located with the transducer and a calibration table stored in memory. As temperature change is sensed, the calibration table is accessed to select an updated excitation frequency.
However, such an approach requires the collection and processing of temperature information. Also, the manner and amount of temperature-induced frequency drift varies from sensor unit to sensor unit. Thus, separate calibration tables may be required for each sensor unit. Further, the natural frequency may change for other reasons, such as component aging.